1. Field of the Invention
The present invention relates to a liquid crystal display device.
2. Description of the Related Art
In an OCB (optically compensated bend) mode liquid crystal display device, a bend alignment is created in a liquid crystal material, and the tilt angles of liquid crystal molecules near alignment films are varied, thereby varying the retardation of a liquid crystal layer. The OCB mode is one of display modes which can realize a high response speed and excellent viewing angle characteristics. In recent years, attention has been paid to the OCB mode.
In the OCB mode liquid crystal display device, as described above, it is necessary to create a bend alignment in the liquid crystal material. However, in an initial state prior to power-on, a splay alignment is created in the liquid crystal material. The reason for this is that the splay alignment is, inherently, more stable than the bend alignment for the liquid crystal material. Thus, when the OCB mode liquid crystal display device is activated, it is necessary to perform a transition the alignment of the liquid crystal material from the splay alignment to the bend alignment.
In order to cause the transition, it is necessary to apply energy, which is greater than a difference in state energy between the bend alignment and splay alignment, to the liquid crystal material. An example of the method for the transition of the liquid crystal material is a method in which electrostatic energy is applied by voltage application to a liquid crystal cell. In this case, since the progress of transition is slow with the application of a voltage corresponding to the state energy difference between the bend alignment and splay alignment, it is necessary, in fact, to apply a very high voltage. Besides, since this transition process tends to be easily affected by the shape of the substrate surface or an electric field distribution, there are cases in which an area with no transition remains in the liquid crystal layer.
In the prior art, in order to solve this problem, the following technique is proposed (Jpn. Pat. Appln. KOKAI Publication No. 2003-280036). Bend patterns with intruding and recessed shapes (hereinafter referred to as “transition nucleus patterns”) are provided around neighboring pixels. A potential difference is applied between the pixel electrodes, and at the same time a potential difference is applied between the pixel electrodes and the counter-electrode. Thereby, a strong strain in the liquid crystal alignment strain is caused to occur in the thickness direction of the liquid crystal cell as well as in the in-plane direction of the liquid crystal cell. Thus, quick transition is made from the splay alignment to the bend alignment.
Also proposed is a technique (Jpn. Pat. Appln. KOKAI Publication No. 2003-280036) in which a transition nucleus pattern is composed of a pixel electrode and a near electrode which is located near the pixel electrode and is connected to a neighboring pixel electrode via a switching element. A potential difference, which corresponds to a pixel signal amplitude, is applied between these electrodes, and at the same time a potential difference is applied between the pixel electrode and the counter-electrode. Thereby, a strong strain in the liquid crystal alignment is caused to occur in the thickness direction of the liquid crystal cell as well as in the in-plane direction of the liquid crystal cell. Thus, while the potential variation due to capacitive coupling between the pixel electrode and neighboring wiring is being suppressed, quick transition is performed from the splay to the bend alignment. In the above techniques, the transition nucleus pattern is provided by overlapping the pixel electrode over the peripheral wiring electrodes, and the aperture ratio and contrast ratio can be kept high.
In the above-described KOKAI No. 2003-280036, in the transition nucleus pattern, the neighboring pixel electrodes are located close to each other, or the pixel electrode and the near electrode, which can apply a neighboring pixel potential, are located close to each other. Further, signal potentials of opposite polarities are applied to these electrodes, and the intensity of a transverse electric field is increased.
However, in the case where the electrodes are located close to each other in the pixel formation plane, it is necessary to consider the restriction by the definition of patterning. For example, in the case of ITO which is used as a transparent electrode material, at least a distance of several μm occurs between electrodes. In other words, in some cases, it is difficult to locate electrodes with less than a distance predetermined by the definition of patterning, and to obtain a desirably intense transverse electric field in the pixel formation plane.
Moreover, in order to realize high-speed transition, it is necessary to set, at predetermined timing, different potentials to the counter-electrode, each pixel electrode and the near electrode, and a complex driving control circuit is needed. Specifically, in the case of the pixel structure using a near electrode in the transition nucleus pattern, two switching elements are needed for one pixel, and it is difficult to simplify the device structure and the driving circuit structure.
Furthermore, the potential difference, which is applied between neighboring pixels or between a pixel and a near electrode, corresponds to a pixel signal amplitude (normally, 10 V or less). Consequently, in some cases, the electric field intensity in the in-plane direction of the liquid crystal cell becomes deficient, and the operation of the transition becomes unstable.